Mercenary Ninja

A Mercenary ninja (傭兵忍者, Yōhei Ninja) is a skilled ninja who is hired to fight for another country other than his own in lieu of money. Because they fight not for loyalty of their own country, all mercenary ninja are also missing-nin. Before the founding of shinobi villages, the majority of the clans in existence at that point were mercenary military forces for hire by whomever.[1]

Trivia

According to Obito Uchiha, back in the days before hidden villages were created, if one side hired the Senju clan, the other would hire the Uchiha clan, and vice versa, as both were the strongest amongst all mercenary clans.[1]

In Shippūden episode 233 a group of mercenary ninja believed Naruto Uzumaki to be a missing-nin and tried to capture him for bounty they assumed was on his head.